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Nelson Pass needs no introduction so none will be rendered. This article partly came about after I read Stereophile's legacy interview with him on-line. There Mr. Pass covered highlights in the evolution of amplifier design - the different measurable aspects that fascinated designers, marketing men and the public and how relative mastery of or concern over one particular design aspect led invariably to others. In short, a time travel synopsis of what, in his mind, was important to designing a good-sounding amplifier.

Because more than 15 years had passed since that interview, I solicited Nelson for a follow-up, a kind of "what have you been up to since" piece. Having already promised exactly that to Stereophile, he proposed instead a think piece on distortion and negative feedback. The latter is both a contentious subject with audiophiles and a subject he had of late given much new thought to. In fact, the latest F5 amplifier is the first under his kitchen-table First Watt brand to apply feedback in a somewhat unconventional way. As my review of it describes, it suffers none of the sonic penalties usually associated with NFB at least by SET guys. As one of the latter, this subject held a lot of personal relevance in general as well as trying to understand just why I like the F5 so very much. So I welcomed his proposal with open ears. Without further ado, the pen now moves into the hands of Nelson Pass with a sincere thank you for his time and generosity. - Ed.

Audiophiles seem to revel in minor controversies - vinyl vs CDs, tubes versus solid state, capacitor, wires, magic dots... and negative feedback.

At one extreme, the position is that "feedback makes amplifiers perfect". At the other extreme, "feedback is a menacing succubus that sucks the life out of the music, leaving a dried husk devoid of soul".

The former viewpoint usually belongs to so-called 'objectivists' who have a fine appreciation for electronic theory and measurements. Their opposites would be the 'subjectivists' who emphasize the listening experience and often own tube amplifiers. Accusations are occasionally made that objectivists can't hear, and conversely that subjectivists hear things that aren't there. This being the entertainment industry, I hope everyone is having a good time.

Feedback is a very large subject and I am going to limit myself to some simple tutorial comments and a discussion of phenomena associated with complexity in distortion created by nonlinear gain stages, negative feedback and the audio signal. Taken singly, these phenomena seem simple enough but when they interact, they create distortions out of proportion to what you expect from the specifications found in product brochures.

There are linear and nonlinear forms of distortion. Linear distortions affect the amplitude and phase of audio signals but don't show up on harmonic distortion analyzers as added frequency components that weren't there in the first place. Tone controls are a good example of circuits with linear distortion.

Nonlinear distortions are those which add new frequency components to the original signal, either as harmonic multiples of the original frequencies or as sidebands resulting from their nonlinear interaction between the original frequencies. Nonlinearities are often deliberately created in musical instruments themselves but they are unwanted in music reproduction. We will be talking about nonlinear distortions.

We use negative feedback in audio amplifiers to stabilize the gain, increase the bandwidth, lower the output impedance and lower the nonlinear distortion. It is the aspect of reducing the distortion which tends to generate the most controversy - negative feedback is very successful in lowering distortion to very tiny numbers as measured by distortion analyzers.

As Mr. Spock said, "Instruments only measure what they were designed to measure." * Given the complaints of audiophiles over the sound of high-feedback type amplifiers, it is reasonable to examine nonlinear distortion in greater depth than is possible with a single number.


* Star Trek, Episode #7, "The Naked Time"